1. Field of the Invention
The present invention relates to a rotary head device for a recording and reproducing apparatus used for recording and reproducing video or audio signals.
2. Description of the Related Art
In high-density recording using a magnetic recording and reproducing apparatus, narrow tracks on a recording medium must be scanned at high density by means of a magnetic head. In a recording and reproducing apparatus which uses a magnetic tape as the recording medium, the tape is generally subjected to helical scanning as a reliable, high-speed scanning method.
The recording and reproducing apparatus, employing the helical scanning method, is provided with a rotary head device. The head device mainly comprises a columnar rotary drum, a magnetic head mounted thereon, and a columnar stationary drum coaxial with the rotary drum. The magnetic tape is run in its longitudinal direction in a manner such that it is wound around the respective peripheral surfaces of the two drums at an angle to the axis thereof. At the same time, the magnetic head is rotated together with the rotary drum at high speed. Thus, the magnetic tape is obliquely scanned at high speed by means of the magnetic head. In this case, a scanning path formed on the tape while the single magnetic head makes one revolution starts at one side edge of the tape and terminates at the other. Part of the scanning path between its starting and terminating points is used for recording or reproducing.
Presently, the helical scanning method is widely used in video tape recorders (VTR) for teaching and home use, digital audio tape recorders (DAT) of the rotary-head type.
The in-surface magnetic recording system is a modern practical magnetic recording system. According to this system, a magnetic layer of a magnetic recording medium is horizontally magnetized by means of a ring-shaped magnetic head. This recording system is also used in a recording and reproducing apparatus of the rotary-head type, represented by the aforesaid helical scanning system. However, the in-surface magnetic recording system is subject to a so-called self-demagnetizing effect such that adjacent magnetic domains on the magnetic tape cancel one another. If the width of the magnetic domains on the tape is reduced to increase the linear recording density, the self-demagnetizing effect is enhanced in proportion. Thus, this effect constitutes a primary factor to restrict the recording density.
Meanwhile, a vertical magnetic recording system was proposed in 1977 by S. Iwasaki et al. According to this system, a linear recording density much higher than in the case of the in-surface magnetic recording system is achieved by vertically magnetizing a recording medium by means of a head for vertical magnetic recording (S. Iwasaki et al., IEEE Transaction on Magnetics, MAG-13.5, p. 1,272 (1977)). The head for vertical magnetic recording, which is composed of two magnetic poles, main and auxiliary poles, scans the recording medium while the medium is held between the two poles. With use of this head, the magnetic recording medium can be magnetized in the direction perpendicular to its surface. Accordingly, adjacent magnetic domains of the recording medium enhance the intensity of one another, so that self-demagnetization cannot be caused. Thus, the higher the recording density, the higher the efficiency will be. The head for vertical magnetic recording of this type, which is called a single-pole head of an auxiliary pole excitation type, is one of magnetic heads best suited for the vertical magnetic recording system.
There is an urgent demand for high-density recording especially in the field of VTRs which, among other recording and reproducing apparatuses requiring high recording density, handle video signals which essentially contain a large amount of information. Thus, the vertical magnetic recording system is intensely required to be applied to the VTRs. When using the head for vertical magnetic recording, such as the single-pole head of the auxiliary pole excitation type, in a VTR, however, high-speed scanning must be effected in a manner such that the magnetic tape is held between the two magnetic poles. It is very difficult, therefore, to apply the head for vertical magnetic recording to the conventional helical scanning system.
Let it be supposed that the head for vertical magnetic recording is positively applied to the helical scanning system. In this case, a head element supported on the rotary drum by means of a head supporting member may be opposed to another head element fixed to the rotary drum so that these two head elements are used individually as the two magnetic poles These head elements, along with the rotary drum, may be rotated at high speed, whereby the magnetic tape is scanned so that it is held between the two head elements. In the case of the helical scanning system, the magnetic tape is wound around the rotary drum at an angle to the axis of the drum, so that the head elements can scan the magnetic tape from the upper edge side thereof to the lower edge side. While the rotary drum is making one revolution, however, the junction of the head supporting member and the rotary drum is bound to be in contact with the upper side edge of the tape. If the drum is rotated in this state, the magnetic tape is inevitably brought into contact with the supporting member, so that it cannot travel smoothly. This is an essential problem which cannot be avoided however long the supporting member, compared with the width of the magnetic tape, may be or even though the way of winding the tape on the rotary drum is changed. Thus, it is very difficult to perform the vertical magnetic recording by combining the head for vertical magnetic recording with the helical scanning system.
The photomagnetic recording system is another recording system of which ultrahigh-density recording can be expected. This system is already in the stage of practical use in the case where a disk is used as the recording medium. In a typical photomagnetic recording and reproducing apparatus using a disk-shaped recording medium, a photomagnetic disk is rotated at high speed by means of a spindle motor. An optical head and a magnetic head are arranged facing each other with the photomagnetic disk between them. The magnetic head, which is used to generate a magnetic field, magnetizes the surface of the photomagnetic disk. The optical head is provided with a laser beam source, optical system, photodetector, etc. At the time of recording operation, the optical head applies heat to a recording region from the rear side of the disk, thereby selectively subjecting a facing portion of the disk to a magnetic reversal. At the time of reproducing operation, moreover, the optical head detects the polarization angle (Kerr rotational angle) of a laser beam reflected by the disk, and fetches a regenerative signal.
Photomagnetic recording can be effected by means of this arrangement when the disk-shaped recording medium is used. When the photomagnetic recording is applied to the helical scanning system using a tape-shaped recording medium, however, the recording medium must be held between the optical head and the magnetic head, thus arousing the same problem as in the case of the vertical magnetic recording.
As described above, the helical scanning system used in the conventional recording and reproducing apparatus of the rotary-head type is an outstanding system in which a flexible tape-shaped recording medium can be scanned relatively stably and with high density. It is very difficult, however, to apply this system to the vertical magnetic recording system or photomagnetic recording system in which the recording medium is held between two heads for recording or reproducing, since the medium will interfere with the head supporting member for holding one of the rotating heads.